A relay node (RN) is used to receive and re-transmit/forward signals intended for user equipments (UEs) in a mobile network. A number of UEs can be served by a relay. The primary objective of the relay node is to enhance the coverage in the uplink and downlink.
In the Long-Term Evolution (LTE) specifications developed by the 3rd-Generation Partnership Project (3GPP), the relay architecture comprises a RN connected wirelessly to a “donor cell,” which is served by a “donor base station (known as a donor eNode B, or DeNB, in 3GPP terminology). The wireless connection between the RN and the DeNB is referred to as the radio backhaul link. UEs connect to the RN via a radio access link that is identical (at least from the UE's perspective) to the radio access link used by UEs to access a conventional eNB.
In LTE, the backhaul link (DeNB-RN link) and access link (RN-UE link) are termed as the Un and Uu interfaces respectively. The connection between different nodes when RN is used is shown on FIG. 1. In FIG. 1, UE 110 is connected to RN 120 via the Uu interface, while RN 120 is connected to eNB 130 (the donor eNode B) via the Un interface, eNB 130 is connected to the Evolved Packet Core (EPC) 140 using the standard interfaces developed by 3GPP.
In practice, the RN may be fixed or movable. Both of these types of RN are wireless RN in that they receive and transmit wireless signals over the access and backhaul links. A “fixed” RN is stationary or immovable. For simplicity, the generic term RN or relay is used throughout this document. In some cases, a movable RN may be implemented as a standalone mobile relay, i.e., as a wireless device dedicated to operation as an RN. In other cases, a wireless terminal that is capable of providing end-user services, whether to a user or to a machine, such as in a machine-to-machine context, may also act as a relay. In some cases, a mobile relay may be deployed in a moving vehicle such as a bus, train, ferry, or the like, in which case it may primarily serve UEs that are aboard the movable vehicle. However, a moving mobile relay may also serve UEs in surrounding areas.
The illustration in FIG. 1 depicts a single hop relay architecture, the details of which are currently being specified by 3GPP for LTE networks. Currently, LTE does not specify multi-hop capability for relays. However, one extension of the relay technique is to deploy multiple relay nodes between the served UE and the eNB. Multi-hop relays are well known in other communications contexts. Information sent to the UE from the fixed network “hops” from the donor base station through multiple RNs, until it reaches the UE. In a similar manner information sent from the UE traverses multiple RN on the way to the donor base station. It should be noted that a multi-hop relay system may comprise any or several of the various types of relays discussed above, including fixed relays, movable relays or a combination of fixed and movable relays. For example, in a two-hop relay deployment scenario, a fixed relay might be used for the first hop while a movable relay is the used for the second hop, or vice-versa.
FIG. 2 illustrates the multi-hop relay concept based on two hops, as it might appear in an LTE network. In the example system illustrated in FIG. 2, UE 110 communicates with RN 210, over the Uu interface. RN 210 in turn communicates with RN 220, which finally communicates with DeNB 130, over the Un interface.
A single- or multi-hop relay system can be employed in any system, e.g., a High-Speed Packet Access (HSPA) network (i.e., UTRA FDD and UTRA TDD), GSM (including GERAN/EDGE), 3GPP2 CDMA technologies (e.g., CDMA2000 and HRPD) or a multi-radio-access-technology (multi-RAT) carrier aggregation (CA) system such as HSPA-LTE CA, etc.
Relay architectures similar to those shown in FIGS. 1 and 2 may also be employed in other technologies such as in HSPA FDD/TDD, GSM/GERAN, CDMA2000/HRPD, WiMax etc. However the relay architecture may be slightly different in other technologies. In general, a RN is connected via backhaul link to any type of donor base station (e.g., belonging to HSPA, GSM, CDMA200, WiMax technology etc). For example, FIG. 3 illustrates a possible relay architecture for an HSPA network. In this figure, a UE 310 is connected to an RN 320 that supports HSPA services. RN 320 is connected to the Node B 330, which in turn is connected to a radio network controller (RNC) 340, via the lub interface.